There are a lot of things that are personal information. Some people may share plans for kids, but I don't tell my family to avoid pressure and my work to avoid problems. Most people won't widely share about infertility, plans to adopt, miscarriages, etc. (After college,) most people don't share information about what happens between the sheets. Most people keep health information to themselves as well, sometimes sharing with friends and family, but not always and often not with coworkers because it could be used against you.
Money just happens to be one more item on this list too. It's not necessarily part of our public life. People can get jealous of friends or coworkers if they know too much information, resentful, etc. I share some finance information with family (such as salary), but even they don't know exactly how much we've saved.
That said, I'm not sure why you'd have difficulty writing to ask about a disbursement of a promised sum. Perhaps that was more to the uncomfortableness of following up on an unfullfilled promise than the uncomfortableness of a money discussion.